Generally, heat sinks are mounted to an outer surface of an integrated circuit (I.C.) package to facilitate the removal of heat from the integrated circuit contained therein. Most heats sinks are thermally conductive and have a plurality of fins to provide a large surface area, which allows heat to be more efficiently dissipated by natural or forced air flow. Heat sinks are connected to I.C. packages in a variety of methods.
For example, a heat sink may be attached to an I.C. package with a sheet of thermally conductive adhesive film, which is placed between the flat surface of the package and the heat sink. The adhesive film is then heated while the package and heat sink are pressed together, thereby bonding the package and the heat sink together. Such a solution is relatively inexpensive, fills any air pockets between the two surfaces, and requires few design restrictions of the package or heat sink other than size and shape conformity. The drawbacks to such a solution include reduced thermal conductivity and migration of the adhesive film over time. Also, different thermal expansion ratios of the heat sink and the package cause mechanical stress on the package. This stress in combination with the curing and hardening required to form the bond leads to decreased mechanical stability. Thus, the heat sink may become loose under shock or vibration. Another drawback to this method is that when the I.C. goes bad or is replaced, the heat sink must be scrapped with the I.C.
Another heat sink attachment method is to add a stud to the heat spreader on the I.C. package and to screw the heat sink to the package using a nut and washer. This method requires a through hole in the heat sink. This method of attachment offers a high compression force between the mating surfaces, thus offering good thermal conductivity. The stud or screw are reliable under shock or vibration. And there is no migration of the fastener over time. However, the assembly torque process results in high stress to both plastic and ceramic packages. This is generally considered one of the most expensive attachment methods due to the secondary operations required in the manufacture of both the heat spreader and the heat sink to construct the stud and through hole. Also, the assembly process requires greater quality control to prevent and catch cracked packages.
There are also a variety of different methods known to attach heat sinks to packages by means of clamps or other equivalent spring loaded systems. The clamps are generally a wire or a sheet metal part made of semi-hard or spring hard materials. The force to ensure the thermal conductivity is provided by the spring load of the clamp. These systems are normally easy and inexpensive methods of attaching a heat sink to a package with very little process control or tools required for attachment. However, each package must have a special clamp designed to match its size and shape. And some solutions require either specially designed packages or printed circuit boards or both. Moreover, the force of each clamp is limited, thus limiting the thermal conductivity obtained. Also, not all solutions ensure a reliable attachment.
With regards to fan heat sink assemblies, typically this assembly is accomplished using screws to attach the fan to the heat sink. This method is expensive because the heat sink must be custom designed with four threads, usually between the fins in either of the four corners, to accommodate assembly with screws through the standard mounting holes on the four corners of the fan. Adding the threads to the heat sink is a very expensive process. Thus, if a heat sink requires special design to fit the particular size and shape of the package to be attached to (i.e. flat heat sink for thermal adhesive bond, through hole or screw for screw/nut fastening, or clamps and hinges for clamp or spring attachment, etc.) and special design depending on whether or not a fan will be attached to it, the slug or heat spreader requires special design depending on the method of attachment and the size of the package, and the package/printed circuit board requires special design depending on the type of heat sink/fan assembly--suddenly the simple task of heat dissipation becomes extremely expensive and design intensive. Accordingly, a simple, inexpensive method of heat sink/fan assembly attachment is needed, which is more standardized without sacrificing the heat dissipation ability of the system.